The use of LED-based lamps is becoming more widespread in home and office environments, since LEDs are efficient and can be realized in a wide range of designs and to deliver precise color temperatures. If an LED-lamp is to be connected to an already installed dimmer, it must be compatible to it. Dimmers of the type used between a power supply and a light source are generally leading-edge or trailing-edge phase-cut dimmers. These work by “cutting off” or suppressing a portion of a sinusoidal mains signal in order to reduce the input power to the light source, either at the beginning of a sinusoidal half-wave (leading edge) or at the end of a sinusoidal half-wave (trailing edge) of a full-wave rectified voltage signal. By ‘removing’ a portion of the input voltage to the lamp, less energy is passed to the following driver electronics. To ensure correct operation of the dimmer, the holding current of the electronic switch needs to be drawn by the lamp's driving electronics (or ‘driver’) throughout the entire mains cycle. For example, a triac requires a holding current of at least 25-30 mA in order to function correctly. This is easy to achieve by the driver of a lamp comprising an incandescent light source, a halogen light source, etc. However, if an LED (light-emitting diode) lamp is to be operated with an already installed or existing dimmer, it needs to be compatible with the dimmer, i.e. it must be able to cope with the high oscillations generated by the dimmer during the phase edges/cuts and to guarantee a minimum current (the ‘holding current’) over an entire phase. Furthermore, the light output by the LED lamp must be reduced according to the dimming level, i.e. according to the reduced operating power.
A modern LED driver draws a relatively low average current, which is a problem when the LED driver is to be used in conjunction with a dimmer. LEDs are low-power devices, and the trend is towards even lower power dissipation as the efficiency of LEDs increases. This means that the electronic driver draws a significant current level only at the beginning of a mains cycle, and draws a low current during the remainder of the cycle. As a result, it may be difficult or impossible for the driver of an LED lamp to continuously draw the required minimum holding current. This often leads to misfiring of the phase-cut dimmer, and this in turn can result in undesirable visible flicker in the light output by the LED lamp.
One way to address this problem is to incorporate a ‘bleeder’ in the dimmer electronics. The bleeder ensures that the driver draws a minimum holding current during the entire mains cycle, independently of the current drawn by the particular LED-driving stage. However, such a bleeder dissipates a significant amount of power, for example in the range of 1.0-2.0 Watt during operation even if there is no dimmer present, or the dimmer is not performing any phase-cut. In some approaches that address the problem of unnecessary power dissipation, digital or mixed-signal circuits are used to detect the presence of a dimmer and/or to detect the activity of a dimmer, and to turn a bleeder on or off as appropriate. However, the need to incorporate such digital or mixed-signal circuitry in a lamp driver adds considerably to its expense.
US 2011/0234115 A1 discloses a LED drive circuit, suitable to be connected to a phase control dimmer. The circuit comprises an edge detection circuit and a current extraction circuit for extracting current from a current feed line for the LED. The value of the current extraction circuit varies in accordance with the detection results of the edge detection circuit. The current extraction circuit may be switched off when no dimmer is present.
Therefore, it is an object of the invention to provide a more efficient and economical way of operating an LED lamp, avoiding the problems mentioned above.